This invention relates to motor vehicle anti-lock brake systems, and more particularly to a control method that adaptively determines exit criteria for terminating anti-lock brake pressure modulation.
A motor vehicle anti-lock braking system (referred to herein as an ABS system) repeatedly releases and re-applies hydraulic brake pressures during conditions of wheel lock to maximize the tractive force between the vehicle tires and the road surface. Typically, the control is initiated in response to insipient wheel lock detection, and is exited when the driver fully releases the brake pedal or the vehicle deceleration vs. wheel slip operating point is in an exit region for at least a predetermined exit time interval. This strategy can unnecessarily delay the termination of ABS control when the driver only partially releases of the brake pedal, which is undesirable, particularly when the road surface coefficient of friction is relatively high. Accordingly, what is needed is method of exiting ABS control that reduces the exit delay, consistent with the road surface coefficient of friction, when the brake pedal is only partially released.
The present invention is directed to an improved ABS control method in which exit criteria for terminating ABS control are adaptively determined based on rate of brake pedal release and estimates of the brake torque and road surface coefficient of friction. In the preferred embodiment, the brake torque and road surface coefficient of friction are estimated based on a periodically updated characterization of the relationship between brake pedal position and vehicle deceleration. In one aspect of the invention, an exit time interval is adaptively adjusted based on the estimated road surface coefficient of friction, and a control value used to determine if ABS control should be exited is adaptively biased toward exiting ABS control based on the brake pedal release rate and the estimated brake torque. In another aspect of the invention, ABS control is exited independent of the timer when the control value indicates that ABS control should be exited and the brake pedal release rate exceeds a threshold, provided the vehicle deceleration is higher than expected, based on the estimated road surface coefficient of friction. In another aspect of the invention, the ABS control is exited independent of the timer if the control value indicates that ABS control should be exited and the brake torque corresponding to the pedal position is significantly less than the maximum possible brake torque, given the estimated road surface coefficient of friction.